The present invention relates to a device for an automatic window winder for automatically lifting and lowering a window glass, and more particularly to a jam detector means for detecting a foreign object jammed between a window frame and the window glass when the latter is moving.
Heretofore, window glass in automobiles has been moved up and down by rotating a crank operatively coupled to the window glass. To eliminate this tedious and time-consuming operation of the crank, there have been proposed and used in recent years various automatic window winders known as "power window regulators" especially in expensive cars.
One example of such automatic window winders includes a reversible driver motor (hereinafter called "motor"), a changeover switch for lifting or lowering the window glass, and two one-shot multivibrator circuits. Each of the one shot multivibrator circuits energizes the motor for a fixed period of time with a trigger pulse for lifting or lowering the window glass.
With the above automatic window regulator, however, the motor is energized for the fixed period of time irrespectively of the position of the window glass prior to being moved. Therefore, the motor will continue to rotate after an upper edge of the window glass hits the window frame and the motor is abruptly stopped if the window glass starts moving upwardly from a partly-open position. The same condition holds true when the window glass is to be lowered. Such forced stoppage of the window glass, if repeated, will shorten the service life of the motor.
When a passenger's hand or neck or another body part happens to be jammed between the window glass and the window frame while the window glass is moving upwardly, the jammed body part is continuously pinched until the motor is de-energized. This is quite dangerous as the pinched hand or neck is highly likely to get injured.
To remove the above shortcoming, there have been proposed various power window regulators having means for detecting a foreign object jammed between the window glass and the window frame upon movement of the window glass.
FIG. 5 of the accompanying drawings illustrates the waveform of current flowing through the motor when it moves the window glass. Designated at (a) is the current flowing when the motor is started, (b) the current flowing when the motor is rotating normally, and (c) the current flowing when the window glass is blocked by a jam. As shown in FIG. 5, the motor current becomes abruptly higher upon locking of the window glass than when the motor operates normally. Based on this characteristic curve, there has been employed a method of cutting off the motor current when the motor current exceeds a reference level whereupon the jam detecting means determines that the window is locked. Since, however, the motor current rises sharply both when it is started and when the window glass is locked, it is necessary to provide a means for discriminating the window locking from the motor starting, resulting in a complex circuit arrangement.
Another method of cutting off the motor current is to monitor the rotational speed of the motor and detects a reduction of the motor speed at the time the window glass is locked, the motor current being cut off by a detected signal indicative of such a speed reduction.
According to each of the speed and current detecting methods, as described above, an absolute quantity is measured and a window-locked condition is determined when the measured absolute quantity deviates from a reference level to stop the motor. However, the motor current or speed is liable to undergo variations due to warpage of the window frame, a change in ambient temperature, and a discharge-induced voltage drop across the car-mounted battery. The reference level for determining whether the window glass is locked or not has to be set to a considerably high level for the motor current or to a considerably low level for the motor speed. This level setting fails to provide sufficient safety.